Hydrolysed proteins have been known in food systems for centuries in the Far East in the form of soy sauce which traditionally has been prepared by enzymatic hydrolysis which requires a long period of time, usually several months, for preparation.
About 100 years ago, a more rapid method of hydrolysing proteins for producing flavours was developed using hydrochloric acid in which the time required is only a few hours. In this acid hydrolysis process, vegetable plant or animal proteins derived from corn, soy, wheat, rice, yeast, peanut or casein are commonly used as starting protein sources and are usually obtained as a result of the separation of the protein fraction during milling of grains or following solvent extraction of oils. The protein contents of these raw materials may range from 40% to 90% with a general average of about 60%. Normally, the protein source is hydrolysed with hydrochloric acid having a concentration of about 20% by weight at a temperature from about 120.degree.-135.degree. C. over a period from about 5 to 8 hours and at elevated pressure up to 30 psig (2 bars). Following hydrolysis, the slurry is neutralised with a suitable alkaline material such as sodium hydroxide or sodium carbonate to a pH from 5.0 to 5.3 and the residual unhydrolysed material (lignin, humin) filtered out. The slurry may be decolourised prior to filtration or the filtrate following filtration may be decolourised by conventional means e.g., activated carbon, absorption resins. Following filtration of the unhydrolysed material, the dilute liquid can be concentrated to precipitate part of the salt formed and then refiltered. Afterwards, the liquid which contains about 42% solids may be further concentrated to pastes or dried by any conventional means such as spray drying or vacuum drum drying to powdered products. Thus, acid hydrolysed proteins can be in either liquid, paste or powder form and are composed mainly of amino acids and salt resulting from the acid catalysed breakdown of peptide bonds present in edible proteinaceous materials. The flavour of the product will, of course, vary depending upon the type of source of protein used as the raw material.
However, the use of hydrochloric acid in the hydrolysis of proteins leads to the formation in side reactions of certain undesirable chlorinated by-products such as .alpha.-chlorohydrins. There are various possible processes which may be considered for reducing the quantities of these compounds. For instance, it would be possible to use a starting material which does not contain any fats, since glycerol is one of the precursors which enables chlorohydrins to be formed with hydrochloric acid. However, on the one hand, starting materials such as these are more or less commercially unobtainable and, on the other hand, would significantly modify the organoleptic qualities of the flavour. In another possibility, hydrolysis could be carried out with a chlorine free mineral acid such as sulfuric acid or phosphoric acid. However, such a modification of the traditional process would also have adverse affects upon the organoleptic qualities of the flavour obtained.